The present invention relates to an optical tracking system that tracks the lateral displacement of an object. In particularly, the present invention relates to a hybrid optical tracking system that tracks the lateral displacement of a subject""s eye during a laser refractive surgery.
In a recent patent application entitled xe2x80x9cOptical Tracking Devicexe2x80x9d and now U.S. Pat. No. 6,179,422, a tracking device is described to employ two scanning beams to scan across a reference mark affixed on an object to be tracked. In an embodiment of eye tracking, the device projects two beams scanning across the limbus at 12 and 3 o""clock positions, respectively.
Two configurations have been described in U.S. Pat. No. 6,179,422. The first one is of open loop, in which the scanning probe beam does not follow the movement of the eye. The second one is of close loop, in which both the surgical laser beam and the probe beam follow the movement of the tracked eye.
The advantage of the open loop configuration is its simplicity and its feasibility to separate the position-sensing device from the optical assembly for the surgical laser beam. Its disadvantage is a limited tracking range due to the curved nature of the limbus, which is the tracking mark for the position-sensing device. The movement detection along two orthogonal directions is no longer independent in an open loop configuration when the probe beams have significant displacement with respect to the curved mark such as the limbus.
In contrast, the close loop configuration can have much larger tracking range while having both the probe beam and the surgical beam deflected via a common beam steering module. The movement detection along the two orthogonal directions is basically independent in a close loop configuration because the probe beams have no significant displacement with respect to the limbus. On the other hand, using a common beam steering module for both the surgical and the probe beams introduces a couple of limitations. First, it requires a more complex optical assembly for the surgical laser beam. Second, it requires a bigger mirror for the common beam steering module, while a bigger mirror means a slower response.
In this application, a hybrid configuration is contemplated to obtain an eye-tracking system having combined advantages of open loop and close loop configurations. The eye-tracking system with such hybrid configuration has an optical assembly of the position-sensing device separated from the optical assembly of the surgical laser beam. The position-sensing device can thus be made as a modular device and serve as an open loop device with respect to the whole tracking system. Meanwhile, the position-sensing device itself includes a beam steering module to direct the probe beams to follow the eye movement, and it can thus provide a larger tracking range. In term of its feedback mechanism, the position-sensing device itself is, therefore, a close loop device.
In a preferred embodiment, the hybrid tracking system consists of a position-sensing device, a system computer, and a first beam steering module. The position-sensing device detects the eye movement and produces x-y position signals of the eye. The system computer reads in the position signals and generates a control signal to the first beam steering module. The first beam steering module thus steers a surgical laser beam to follow the eye movement.
In the preferred embodiment, the position-sensing device comprises a first and a second scanning beam generators, a second beam steering module, an optical assembly, a first and a second photo detectors, a processing electronics, and a control unit. Each scanning beam generator produces a scanning probe beam. The second beam steering module directs the first and second scanning probe beams on to the eye such that the two beams scan repetitively across the limbus at 12 and 3 o""clock positions, respectively. The optical assembly focuses scattered light of the probe beams on to respectively the first and second photo detectors. As each probe beam scans across the limbus, the corresponding detector records a sharp change in the scattered light signal. The timing of this sharp change in the detector signal indicates the relative position between the scanning probe beam and the limbus. The processing electronics measures this timing with respect to a reference time position to produce a delay time Td. The control unit analyzes this delay time Td to generate a driven signal Vd to steer the second beam steering module such that the delay time Td is kept around an initial value Td0. By this way, the scanning probe beam follows the movement of the eye, and the position-sensing device works as a close loop device.
When the response speed of the position-sensing device including the second beam steering module is fast enough to follow the eye movement, the driven signal Vd is proportional to the displacement of the eye. This signal Vd can then be used directly as x-y positioning signals of the eye. If the second beam steering module is slower than the involuntary eye movement, the eye displacement with respect to its initial position can be determined by measuring simultaneously the angular position xcex1 of the second beam steering mirror and the delay time Td. The control unit analyzes xcex1 and Td for the two probe beams and generates x-y positioning signals of the eye.
The surgical system computer can then use these x-y-positioning signals to guide the surgical laser beam to follow the eye movement. In this way, the position-sensing device feeds one-way signals to the surgical laser system and the system thus works in an open loop configuration.
Accordingly, an advantage of this hybrid-tracking system is its optical and mechanical separation of its position-sensing device from the other part of the tracking system and thus enables to design the position-sensing device into a modular device.
Another advantage of this hybrid-tracking system is its close loop configuration in detection, which enables a large tracking range for a moving object with a curved reference.
A further advantage of this hybrid-tracking system is its position detection scheme, which makes fast eye tracking (i.e., positioning detection) achievable even a relatively slow beam steering module is used in the position-sensing device.